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1. Field of the Invention
This invention relates generally to computer systems, and more particularly provides a system and method for digitally marking a file with a removable mark.
2. Description of the Background Art
Real world data, such as audio and video data, are recorded on various mediums. Older technology, such as analog audio and video tapes, analog photographs, television and radio signals, etc., record real world data in analog form. Analog data on an analog medium is difficult to modify undetectably.
However, modem systems use digital technology. Examples of such systems include digital audio recordings on tape or CD""s or computer memory or hard disks, digital still or motion pictures on tape or DVD""s or in computer memory or hard disks, digital still or motion pictures or digital audio data sent over the phone lines or on computer networks, digital audio signals sent over the radio waves in devices such as digital microphones or cordless telephones. Because of the ease of data manipulation of a digital file, it is difficult to detect whether recorded digital data is the original real world data or a modified version.
Some prior digital imaging systems add a xe2x80x9cdigital watermarkxe2x80x9d to an image. A user typically selects a region in the image in which to replace the original real world data with a word or symbol. The digital watermark is designed to survive many types of digital editing and alteration. However, the digital watermark irrecoverably sacrifices some of the real world data and merely provides evidence of the origin of the digital work. Further, one hoping to authenticate the contents of the image cannot reasonably assume by locating the watermark in the image that the image contains only the original real world data.
Other prior digital systems append code to the real world data file containing the real world data. However, each system uses a different protocol and format. Thus, when converting a digital file between formats, the appended code is typically discarded as unnecessary. A user can thus modify the contents of the data file undetectably.
Therefore, a system and method are needed for encoding into a digital file a mark, which does not irrecoverably sacrifice any of the original data, which is not lost by converting the file data between different protocols and formats, and/or which may identify the original content.
The present invention provides a system and method for marking any data file that contains a xe2x80x9cflatxe2x80x9d area, i.e., a set of data values wherein all the data values vary within a certain amount of each other. The length of the word can be any number of bits greater than one (1). In a digital picture, examples of flat areas may include a set of navy blue pixels, a set of fire-engine red pixels, etc. In an audio recording, examples of flat areas may include a set of sound bits of a particular volume. Flat areas are almost always found in recordings or transmissions of natural information, e.g., a still picture, a motion picture, an audio recording, an audio transmission, a measurement of radio signals or other forms that occur in nature, etc. Examples of flat areas in real life may be found in data representing a sky, a tree, a speaker""s voice, a bass guitar solo, etc.
An example system and method place a content-identifying mark within the flat area of the data file. If alteration is performed on the original data, either the mark will not match the data or the mark itself may be damaged. The system and method attempt to modify the original data as little as possible, enable removal of the mark from the data, and, if unmodified, enable return of the original data. This would help ensure the integrity of the digital data involved and has many applications including ensuring the validity of digital photographs, digital motion pictures and digital voice or sound recordings for use as evidence in a court of law. Another use ensures the validity of scientific research data recorded by instruments or the validity of data transmitted over computer networks, radio transmission or other means.
The system comprises an encoder and a decoder. The encoder includes a target area locator for locating in the digital data a flat area having a flatness value n, and includes a marker for using the flatness value n to encode a mark into the flat area. The decoder attempts to extract a mark that from digital data. The decoder includes a mark area locator for using a flatness value n to search digital data a mark, an unmarker coupled to the flat area locator for decoding a possible mark and for using the flatness value n to replace the possible mark with possible original data, and an authenticator coupled to the unmarker for examining the possible mark for accuracy.
The method comprises an encoding method and a decoding method. The encoding method encodes a mark into digital data, and includes locating in digital data a flat area having a flatness value n, and using the flatness value n to encode a mark into the flat area. The decoding method attempts to extract a mark from digital data, and includes using a flatness value n to search digital data for a possible mark, decoding the possible mark, and examining the possible mark for accuracy. The method may also include using the flatness value n to replace the possible mark with possible original data.
The system and method may advantageously detect altered data to a configurable level of confidence. The system and method may advantageously allow the original digital data to be completely restored to it""s unmarked original state. The system and method may mark the digital data in a subtle way, so that the mark is not easily noticed if displayed in it""s marked state. The system and method may advantageously require no special preparation of the data. The system and method advantageously can be used with various types of digital data. The system and method advantageously require no human intervention.